Line level signal to microphone input adapter

Sometimes there is need to convert line level signals to such signal
that it can be connected to microphone input.
Because the line level signals are typically in range of 0.5..2V and
the microphone signals are in millivolt range, quite much attenuation
is needed to match the signal levels. This means that typically
you will need 40-50 dB of attenuation.

40 dB PAD

Consumer audio line (-10dBu) level to microphone
matching can be done using a "40 dB L-pad attenuator".
This circuit is indeed quite simple:

20 dB PAD for line to electret microphone input

This circuit is designed to interface consumer (-10 dBu)
signals to microphone input that is designed for two-were electret
microphone capsules. This circuit provides around 20 dB of
signal attenuation (typically enough to make thigns work,
if more attenuation is needed use higher R1 value) and
blocks the DC bias used in the electret microphone input
to get to the line signal side (C1 does this).
This circuit look like around 1 kohm signal source to
at the mic side, whic matches pretty well to electret
capsule characteristics (similar impedance and provides
DC path for bias to go to ground).

This equation is quite accurate when the impedance
of the mic input where this circuit is connected
has much higher impedance than the resistance of R2.
Typical microphone inputs in equipments have input impedance
of 1.5 kohm or higher (which is much higher than 100 ohms).

Component selection tips:

R2 should prefeerably be something between 10 ohms and one kilo-ohms

R1 should be from few kilo-ohms to few hindred kilo-ohms..

Those value ranges generally works bets for audio applications.

Constructing the circuit

The circuit itself can be built to a small metal box or even
can be fitted inside an audio connector. The resistors in this
circuit do not need more than 1/4W power rating (even lower ratings will do).
I would recommend using metal film resistors in this circuit, because they
are less noisy than cheap carbon film resistors.

I don't recommend building the circuit to a plastic box because
we are playing with low level microphone signals, so the interference
from nearby equipments or cellular phones can be a signnificant if
you build this circuit to a plastic box or without any box. If you
plan to use the circuit in any serious audio system do yourself a
favor and build a circuit to a good metal box.

What to do if I get humming noise when I connect this to my system ?

Unbalanced microphone inputs of audio equipments are very sentitive to all
noise in the system, especially when you connect something else than
a flowating microphone to them. Usually at short distances there
are no problems in usign the adapter.
If you get any 50/60 Hz noise problems, put an audio isolation transformer
to teihter input or output side of my circuit.